Many of biocatalysts such as enzymes and yeasts are prone to undergo destruction of the molecular conformation and deterioration of the activity, that is, catalytic ability, under the influence of temperature, pH, solvent, or intermolecular electrostatic repulsion. Therefore, when a biocatalyst is stored or utilized in a biocatalytic reaction, it is necessary to maintain the conformation of an active site and the conformation of amino acid residues. Regarding a method for long-term storage of a biocatalyst, there are known a freeze drying method of storing a biocatalyst in a powdered form, and a freeze preservation method of storing a biocatalyst by dissolving the biocatalyst in a solution under cryogenic conditions at a low concentration.
Generally, storage as a solution is desirable in view of the convenience of operation; however, in the case of a freeze preservation method, special apparatuses are required, and also, there are a problem that since ice is produced at the time of freezing, the conformation of a biocatalyst is destroyed, and a problem that when a frozen solution is used after thawing, the structure of the biocatalyst changes, and the activity is deteriorated. Furthermore, the storage concentration is, for example, in the case of urease or catalase, generally as low as about 1 to 3 mg/mL, and efficient preservation is difficult.
In order to prevent deactivation of such a biocatalyst and to retain the activity of an enzyme, attempts have been made to add stabilizers. For example, a method of using a polyhydric alcohol such as glycerin or sorbitol for the preservation of uricase (Patent Literature 1), and a method for stabilizing cholesterol oxidase by adding bovine serum albumin and sugars to a solution containing cholesterol oxidase (Patent Literature 2) have been proposed. However, in these methods, there is a problem of deterioration of enzyme activity in relation to the storage concentration, storage temperature, or storage period.
Furthermore, an ionic liquid is an organic salt which consists of a cation and an anion, and generally, ionic liquids composed of imidazolium-based cations or quaternary ammonium cations and various anions are known. Due to their structural features, investigations have been conducted on various applications of ionic liquids. Under such circumstances, it has been reported that enzyme activity is retained by adding an ionic liquid to a reaction solution of an enzyme or using an ionic liquid as a solvent for the reaction solution (Patent Literatures 3 and 4, and Non-Patent Literature 1); however, there is a problem concerning the dissolution concentration and preservability.